This study is based on the hypothesis that: 1. Some thyroid carcinomas are epigenetic in nature, and 2. the expression of the embryonal and adult genome may be influenced by thyrotropin and certain cellular/humoral factors respectively. Both the embryonal and adult genome are sequentially manifested in some primary, poorly functioning human papillary-follicular thyroid carcinoma, which can form metastases composed of normal-appearing and well-functioning follicles, especially in the lung and bones. The same phenomenon was observed in some transplantable papillary-follicular thyroid tumors of the rat when trypsin dispersed cells of these tumors were injected intracardially in the same inbred species of rats. To test these hypotheses, the transplantable thyroid tumors of the rat will be studied in vivo and in cell culture, by determining the effect of TSH on light and electronmicroscopic structure, growth rate (tumor weight in rats - doubling time, colony formation, cell cycle, growth fraction and chromosome composition in cell culture) and thyroid hormonogenesis. Methods will be developed to determine the presence of TSH binding receptors in the cell membrane of the transplantable thyroid tumor of the rat and of the human thyroid carcinoma (obtained from the operating room). It is expected that these studies will contribute to the understanding and therapy of human thyroid carcinoma.